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UDC 621.375: 621.391.6
Wideband WDM Erbium-doped Optical
Fiber Amplifiers for 10 Gb/s, 32-channel
SMF Transmission Systems
VSusumu Kinoshita
VTadashi Okiyama VKaoru Moriya
(Manuscript received March 8, 1999)
We have developed novel wide-dynamic-range WDM optical fiber amplifiers for 10 Gb/s,
32-channel SMF transmission systems. These amplifiers consist of a low-noise preamplifier section, a mid-attenuator for loss compensation of a dispersion compensator and ALC operation, and a post-amplifier section for a high output power. These
WDM amplifiers are also designed to allow in-service channel upgrades and in-service
accommodation of external pump units. We have confirmed the superior performance
of these WDM amplifiers by performing 320 Gb/s (10 Gb/s ××32-channel) transmissions
over 320 km (80 km ××4 span) of SMF, achieving a bit error rate of less than 10-15 for all
channels.
1.
Introduction
Wavelength-division-multiplexing (WDM)
systems have become one of the major solutions
to meet the growing demand for increased network bandwidth brought about by the rapid
growth of Internet and data services. Wideband
optical fiber amplifiers are essential for WDM
trunk-line systems because of their broad gain
bandwidth, low noise, and good high-power characteristics.
We focus on developing WDM amplifiers that
not only satisfy high-capacity requirements but
also make it possible to fully realize the potential
of WDM technology, for example, flexible, lessexpensive, scalable services.1),2) The developed amplifiers are intended for a 1.3 µm zero-dispersion
single-mode fiber (SMF) transmission system.
This is because most of the embedded fiber cable
in use is SMF, especially in capacity-demanding
North America. Moreover, because a WDM
amplifier for SMF requires more functions than
any other WDM amplifier, for example, the
accommodation of a dispersion compensator, the
82
technologies employed for them are applicable to
any other WDM amplifier. SMF has a large chromatic dispersion of about 18 ps/nm/km at 1.55 µm,
so dispersion compensation is indispensable for
high-speed channel rate (10 Gb/s) transmission.
In addition, WDM optical fiber amplifiers must
maintain a flat-gain and control the output power of each channel, referred to as “per carrier
automatic level control (ALC) operation” to compensate for the span-loss variation encountered
in practical systems. Because the signal power is
controlled at the output of the amplifier, the spanloss variation therefore causes power variations
at the amplifier input. The spectral-gain change
caused by input level variations is called “dynamic gain-tilt.” We outline these issues first and then
explain our proposed WDM optical fiber amplifiers which answer the above issues. We also
propose a novel channel upgrade method using
the proposed WDM amplifiers and the standardized Optical Supervisory Channel (OSC) system.
FUJITSU Sci. Tech. J.,35,1,pp.82-90(July1999)
S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
Requirements for Wideband WDM
Optical Fiber Amplifiers
In addition to the features required in a
single-channel optical amplifier (e.g., low noise and
good high-power characteristics), WDM optical
fiber amplifiers must have the following:
1) Broad gain bandwidth for simultaneously
amplifying many signals in 32 wavelengths
2) Gain flatness over a wide input power range
3) Output power controllability of each amplified signal
4) Loss compensation of dispersion compensators
5) Output power stability during an increase or
decrease in the number of channels
6) A smooth upgrade scheme for an increase or
decrease in the number of channels.
Among these requirements, the second and
third are especially important when WDM
amplifiers are used as optical repeaters. In optical-amplifier repeated systems, the output power
of each channel at the amplifier should be kept
within the optimum level window to avoid excessive generation of nonlinear effects and a
deterioration of the signal-to-noise ratio (SNR) as
optical signals travel along the transmission
fiber (Figure 1).
Taking account of their simple and cost-effective configuration, WDM optical amplifiers are
likely to be used to control the total output power
instead of the output power of individual channels. The wide dynamic input range of WDM
amplifiers with the spectrally-flattened gain
therefore becomes essential in the total output-
Output
Input
WDM amplifier
P
P
λ
ALC
ALC: Automatic
level control
Nonlinear effect
ASE accumulation
λ
power control scheme, which allows the same
amplifier to be applied to various span-loss sections. Figure 2 shows the spectral gain of an
erbium-doped fiber (EDF) at various inversion
rates. Owing to the homogeneously broadening
characteristic of the EDF, the spectral shape is
fixed if the average-gain or the inversion rate is
clamped. Thus the combination of gain-clamped
EDFs and passive optical gain equalizers (GEQs)
provides a spectrally-flattened gain over a wide
range of input power. However, in this combination, the total output power varies with the input
power, which means that there is no ALC function. We therefore introduce a variable optical
attenuator to overcome this problem. The aforementioned combination technology fixes the
spectral characteristics and provides attenuation
for the ALC operation. We realize per carrier ALC
operation by total-power ALC operation based on
the number of operation channels. Requirements
5) and 6) relate to the flexibility and scalability of
the WDM system. The ability to perform channel
upgrades without affecting the surviving channels
is an indispensable requirement for reducing the
initial investment and realizing a cost-effective
WDM system.
1
0.8
Inversion rate
1.0
0.6
Relative gain coefficient
2.
0.9
0.8
0.4
0.2
0.7
0.6
0.5
0
-0.2
0.3
-0.4
0.2
-0.6
0.1
-0.8
-1
1450
0.4
0.0
1500
1550
1600
1650
Wavelength (nm)
Figure 1
Window of optimum output level in optically amplified
transmission.
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
Figure 2
Spectral-gain at various inversion rates.
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S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
3.
Key Elements for WDM Optical Fiber
Amplifiers
3.1 Alumina highly co-doped erbiumdoped fiber
EDF is designed to obtain a maximally flattened spectral-gain over the entire signal band.
To flatten the spectral-gain, aluminum (Al) is highly co-doped with erbium (Er). The Al concentration
is optimized because too much concentration produces a gain-dip around 1545 nm. Consequently,
we achieve a flattened spectral-gain characteristic of over 30 nm from 1530 nm to 1565 nm in
which the gain deviation between the maximum
and minimum is reduced to 4 dB from 6.5 dB at a
30 dB average-gain operation. Furthermore, the
efficiency is improved by reducing the background
loss from 20 dB/km to 6 dB/km through optimization of the manufacturing conditions. The
polarization mode dispersion (PMD), which is
another important characteristic, is less than 0.05
ps per 10 m. This value is low enough for highspeed (10 Gb/s) optical network systems.
3.2 Non-mechanical variable optical
attenuator
To increase the dynamic input range of the
WDM amplifier, ALC operation based on a specially developed variable attenuator3) plays an
important role. The variable attenuator consists
of two polarizers and a Faraday rotator as shown
in Figure 3.
Variable
Faraday rotator
Fiber
Fiber
The rotation-angle of the polarization is
controlled by varying the driving current of the
rotator. Because the operating wavelength is
1.55 µm, the most efficient Faraday rotator is made
of iron garnet crystal. However, it is impossible
to control the magnitude of the magnetization continuously under magnetic saturation because iron
garnet is ferromagnetic. On the other hand, if the
magnetization is not saturated, the crystal causes scattering and has magnetic hysteresis. Our
non-mechanical variable attenuator utilizes a
novel method of controlling the Faraday rotation
angle. If the angle between the direction of magnetization and light propagation is θ, the Faraday
rotation is proportional to cosθ. A continuous
change in the magnetization direction is thus possible even under magnetic saturation by the
variable Faraday rotation. A significant feature
of the proposed method is good continuousness
with no scattering or hysteresis. The variable
Faraday rotator is inserted between two birefringent wedges as shown in Figure 3, which is the
same as the scheme used in an inline optical isolator to eliminate the polarization dependence.
The first wedge refracts the input light collimated by a lens, with the ordinary and extraordinary
light being deflected at different angles. After the
Faraday rotation, a part of each light beam couples again to the output fiber by refraction in the
second wedge. The amount of coupling to the output fiber is determined by the degree of Faraday
rotation. Thus, current-driven variable attenuation is realized without polarization dependence.
A current of 0 to 50 mA drives the attenuator from the minimum attenuation (0.8 dB) to the
maximum attenuation (25 dB). Without a fiber
cover, the attenuator is 23 mm long, 25 mm wide,
and 8 mm high.
Lens
Lens
Birefringent wedge
Wide-dynamic-range WDM Optical
Fiber Amplifiers
Figure 3
Non-mechanical variable optical attenuator using Faraday effect.
The proposed WDM optical fiber amplifiers
have the following features:
1) A flat-gain bandwidth over 25 nm for 32
84
4.
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
wavelengths in an ITU-T 100 GHz grid
2) Per carrier grid-to-grid automatic level control (ALC) operation4),5) over a 7 dB input
power range
3) Field-installation of dispersion compensator
by automatic loss compensation5)
4) Flexible configuration allowing the fieldinstallation of external pump units
5) Scalability for the number of wavelengths by
using an ALC/AGC switching scheme6)
6) Simple and cost-effective configuration
Figure 4 shows the configuration of the proposed WDM optical fiber amplifier. The amplifier
consists of a low-noise pre-amplifier, a mid-attenuator for ALC operation, and a post-amplifier for
a high output power. The AGC scheme is employed
in both the pre- and post-amplifier sections in
order to keep the spectral-gain constant over input power variations. Two gain equalizers, GEQ1
and 2, are used to flatten the gain over the entire
signal band of 32 wavelengths (1535.8 to 1560.6
nm) for the pre-amplifier and post-amplifier, respectively. A dispersion compensator (DC) is
employed between the pre-amplifier and the postamplifier. Using the connector interface with the
DC, the amplifier provides the flexible accommo-
dation of many kinds of DCs. The configuration
of two separated and gain-flattened blocks with
DC not only provides a high SNR in each channel
but also makes it easy to fabricate and test the
amplifiers. It also reduces the production list of
the amplifiers by combining different pre- and
post-amplifiers. The variable optical attenuator
(VAT) using the Faraday effect absorbs the spanloss and DC loss variations to realize ALC
operation. When the number of wavelengths is
increased, the amplifier can accommodate external pump units in the post-amplifier during
operation; this is one of the remarkably smooth
upgrade functions of these amplifiers. The optical supervisory channel (OSC) section is attached
to the amplifier section, which demultiplexes the
supervisory channel (λSV :1510 nm) at the input
and multiplexes it at the output. The OSC collects supervisory information of each unit and
carries it downstream. Figure 5 shows a photograph of the amplifier section.
Several kinds of DCs have been proposed.
Among them, a dispersion compensating fiber
(DCF) is promising for its wideband compensation characteristic and stability against ambient
temperature variation. There are two problems
λ sig, N
λ sig, N (1535.8 nm ~ 1560.6 nm)
Input
EDF
EDF
GEQ1
VAT
DC
EDF
Output
GEQ2
Cont.
λsv
Pump
LDs
Output
monitor
Cont.
Pump
LD
External
pump
sources
AGC
AGC
Pump
LDs
λsv
(1510 nm)
ALC
SV
Figure 4
WDM optical fiber amplifier for 10 Gb/s ××32-channel SMF transmission.
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
85
S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
in the application of the DCF. One problem is the
relatively large optical loss (~10 dB) of the DCF
and its loss dependence on the amount of compensating dispersion. The other problem is the
generation of nonlinear effects in the DCF because
of its small mode-field diameter and long length.
This problem restricts the input power applied to
the DCF to less than 0 to -2 dBm/ch.7)
The DCF loss is compensated for in the
pre-amplifier and mid-attenuator section. The
proposed amplifier also overcomes above problems. It realizes ALC operation by automatically
compensating for the DCF loss as shown in
Figure 6 (a). First, the pre-amplifier sufficiently
amplifies the WDM signals at low noise. The variable optical attenuator (VAT) reduces the input
power to the DCF to below the allowed maximum.
At the same time, it controls the input power of
the post-amplifier. Although the detailed control
lines are omitted in Figure 4, the input-level of
the post-amplifier is adjusted to a pre-determined
level (controlled level) in the initializing process.
After initialization, the VAT is controlled mainly
by feedback derived from the input to the DCF in
Figure 4. The control level is determined from
the allowed maximum input power (for example,
-3 dBm/ch) and from the maximum DCF loss in-
Figure 5
Photograph of proposed WDM amplifier.
Controlled output level (dBm/ch)
7
Power
Allowed maximum input
level to DCF (dBm/ch) (-3)
AGC
(14.5 dB)
AGC
(20 dB)
(10 dB)
Input
-17
Pre-amplifier
VAT
DCF
Amplifier position
Output
Controlled level
(dBm/ch)
(-13)
Post-amplifier
(a) Control for DCF loss variations.
Controlled output level (dBm/ch)
7
Allowed maximum input
level to DCF (dBm/ch) (-3)
Power
AGC
(14.5 dB)
(7
AGC
(20 dB)
B)
d
Input
-17
Pre-amplifier
VAT
DCF
Amplifier position
Post-amplifier
Output
Controlled level
(dBm/ch)
(-13)
-10
(b) Control for input power variations.
Figure 6
Level diagrams of proposed WDM amplifier.
86
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
cluding connector losses (10 dB), which gives
-13 dBm/ch. The amplifier therefore realizes per
carrier ALC operation by absorbing the variation
in DCF loss with an input power of less than -3
dBm/ch. Moreover, if the input power to the preamplifier varies by the span-loss variation shown
in Figure 6 (b), the mid-attenuator between the
pre-amplifier and the DCF always absorbs the
variation in signal power.
Figure 7 shows a typical output spectrum
for 32-wavelength amplification. The stability of
spectral-gain and per carrier grid-to-grid ALC operation is shown in Figure 8. Over a 7 dB input
power range (-17 to -10 dBm/ch), the output powers of the 32 wavelengths were controlled to within
7.0 ±0.6 dBm and no major spectral-gain change
was observed. Figure 9 (a) shows the noise fig-
ures of the 32 wavelengths at five points over this
power range with compensation for a 9 dB DCF
loss and with the OSC demultiplexer loss included. Figure 9 (b) shows the maximum noise figures
at these five input powers. Note that even with a
9 dB DCF loss compensation, the maximum noise
figure (NF[Max]) at -17 dBm/ch was only 7.2 dB.
Smooth changing of the number of wavelengths (or channels) is an important requirement
in a WDM system. Usually, for a simple and costeffective configuration, the amplifier controls the
total output power instead of individually controlling the output powers of each channel. However,
the problem with total-power-controlled ALC
operation is that the reference changes when the
number of channels varies. Scalability for the
number of channels is realized by the sequence
25
Input power
(dBm/ch)
23
Gain (dB)
Output power (dBm)
20
0
-20
19
17
15
1535
-40
1532
1542
1552
Figure 7
Typical output spectrum for 32 wavelength amplification.
1545
1555
1565
Wavelength (nm)
1562
Wavelength (nm)
-17
-15
-13.5
-12
-10
21
Figure 8
Stability of spectral-gain and per carrier operation with
input power variations.
-10
-12
-13.5
-15
-17
10.5
8.5
6.5
1530
1540
1550
Wavelength (nm)
(a)
1560
1570
Noise figure (dB)
Noise figure (dB)
14
Input power
(dBm/ch)
12.5
12
NF [Max] (dB)
10
8
6
-17
-15
-13
-11
Input power (dBm/ch)
-9
(b)
Figure 9
(a) Noise figures of 32 wavelengths for various input powers with a 9 dB DCF loss compensation and the OSC
demultiplexer loss included.
(b) Maximums of Figure 9 (a).
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
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S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
described below.6) The sequence requires a new
function in the OSC instead of extra optical components such as an optical spectrum monitor in
the amplifier. A channel upgrade involves changing from ALC operation to AGC operation by
freezing the operation of the variable optical attenuator. During AGC operation, the total output
power is always equal to the total input power
multiplied by a fraction of the net gain, which
means that the AGC mode allows the input power to change by channel adding or dropping
without affecting surviving channels. In the proposed WDM amplifiers, ALC operation is realized
by adjusting the attenuation in the mid-attenuator while each EDF-section operates in AGC mode.
Switching between ALC and AGC is easily done
by controlling the operation of the mid-attenuator.
Figure 10 shows the channel upgrade sequence. The sequence is performed as follows.
1) First, a kind of network management system
(NMS) gives an advance announcement about the
channel upgrade to each amplifier through the
OSC. The announcement also includes information about the new number of channels. 2) Each
amplifier switches its operation mode from ALC
to AGC by freezing the operation of the variable
OS
•Notification of channel upgrade and
the new number of channels
OS
OS
•Channel addition
OS
Stand-by
Clear
ALC
•Switch to AGC operation
•Adjustment of reference voltage
of ALC operation for the new
number of channels
AGC
Finished
ALC
•Switch to ALC operation
Figure 10
Sequence of channel upgrade through optical supervisory channel.
88
attenuator and adjusts the reference voltage of
ALC operation according to the new number of
channels. Then, the amplifiers respond to the
NMS by sending a “Stand-by” signal. 3) After receiving “Stand-by” responses from all amplifiers,
the NMS smoothly increases the power of the added channel or channels to make the AGC operation
follow the increase in input power. The AGC operation allows the input power to be increased
without affecting the output power of surviving
channels. Then, the NMS instructs each amplifier to return to ALC operation. 4) The amplifiers
change to ALC operation based on the new reference voltage, which avoids or minimizes needless
transient responses from amplifiers. Finally, the
NMS finishes the upgrade process after it receives
“Finished” responses from all amplifiers. A channel drop is executed using a similar sequence.
We measured the transient response of one
of the surviving channels when the number of
channels was increased during AGC operation. In
this test we used five wavelengths at input
powers that emulated an addition from 16 wavelengths to 32 wavelengths for an input power of
-15 dBm per wavelength. Table 1 shows the input powers of these wavelengths during the test.
Wavelengths 1536 nm, 1548 nm, and 1560 nm
were set to -8 dBm to emulate the power of five
wavelengths each, and wavelength 1554 nm was
set to -15 dBm to represent a single wavelength.
This gave an equivalent of 16 wavelenths (5 + 5 +
5 + 1 = 16). Then, wavelength 1547 nm was
changed from less than -30 dBm to -3 dBm to emulate the addition of 16 new wavelengths and the
change in wavelength 1554 nm was measured.
The output power response of the 1554 nm signal
was measured through a narrow bandwidth optical filter when the 1547 nm signal was added. The
rise time of the added signal was 5 ms. Figure 11
shows that the output power deviation of wavelength 1554 nm after the addition was as low as
0.22 dBpp.
We have confirmed the superior performance
of these WDM amplifiers by conducting 320 Gb/s
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
WDM transmissions over a 320 km SMF, achieving a bit error rate of less than 10-15 for all
channels.
5.
functions do not limit the usable bandwidth.
Therefore, those technologies can be applied to any
other band within the 40 THz capacity of the transmission fiber with new optically-amplifying fibers.
Conclusion
We described the novel wide-dynamic-range
WDM optical fiber amplifiers we have developed
for 32 × 10 Gb/s SMF transmission systems. These
amplifiers operate by automatically compensating for span loss and DC loss variations based on
information about the number of wavelengths
which is obtained through the OSC. These WDM
optical fiber amplifiers realize per carrier ALC
operation over a 7 dB input power range. We also
described how scalability for the number of wavelengths is achieved using an ALC/AGC switching
scheme.
These WDM amplifiers make it possible to
fully realize the potential of WDM technology, for
example, flexible, scalable, and less-expensive construction of high-capacity transmission systems.
Moreover, unlike the new rare-earth doped optically-amplifying fibers, the technologies we have
developed that relate to new configurations and
Table 1
Input powers during emulation of addition from 16 wavelengths to 32 wavelengths.
Wavelength
(nm)
Input power
(dBm)
1536 1548 1560
-8
-8
5 ms/div.
-8
1547
1554
(added)
(measured)
-30 > ← -3
-15
Channel addition
(16 → 32 chs)
References
1)
2)
3)
4)
5)
6)
∆Po : 0.22 dBpp
7)
Off-level
Figure 11
Change in output power of a surviving channel (1554 nm)
due to an emulated addition from 16 wavelengths to 32
wavelengths.
FUJITSU Sci. Tech. J.,35, 1,(July 1999)
S. Kinoshita, H. Onaka, and T. Chikama :
Large capacity WDM transmission based on
wideband erbium-doped fiber amplifiers.
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S. Kinoshita et al.: Wideband WDM Erbium-doped Optical Fiber Amplifiers for 10 Gb/s, 32-channel SMF Transmission Systems
Susumu Kinoshita received the Dr.
Eng. degree from the Department of
Information Processing, Tokyo Institute
of Technology, Tokyo, Japan in 1987.
From 1987 to 1989 he was with P&I
Laboratories, Tokyo Institute of Technology as a research associate. He joined
Fujitsu Laboratories Ltd., Kawasaki,
Japan in 1989 and has been engaged
in research and development of optical
fiber amplifiers and their applications to
optical networks. He is a member of the Institute of Electronics,
Information and Communication Engineers (IEICE) of Japan and
the Japan Society of Applied Physics.
Kaoru Moriya received the B.S. degree
in Mechanical Engineering from Tohoku
University, Japan in 1978. In 1978
he joined Fujitsu Ltd., Kawasaki and
has been engaged in development of
photonic devices, particularly optical
passive devices and erbium-doped
fiber modules. He is a member of the
Institute of Electronics, Information and
Communication Engineers (IEICE) of
Japan.
Tadashi Okiyama received the B.S.
degree in Electrical Engineering from
Tokyo University of Agriculture and
Technology, Tokyo, Japan in 1980. He
joined Fujitsu Laboratories Ltd.,Kawasaki
and Fujitsu Ltd.,Kawasaki in1980 and
1993, respectively. He has been engaged in research and development of
optical fiber amplifiers for high-speed
optical communication systems. He is
a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of
Japan.
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FUJITSU Sci. Tech. J.,35, 1,(July 1999)